Method and system for detecting special nuclear materials

ABSTRACT

A method and system for detecting special nuclear materials are disclosed. Said method and system detect the special nuclear materials by making use of the photofission characteristic and thermal neutron induced fission characteristic thereof. In one preferred embodiment, the high density and/or high atomic number region in the object to be detected is also detected first as a suspicious region.

This application is a U.S. National Stage Application of PCT ApplicationNo. PCT/CN2009/000498, with an international filing date of 7 May 2009.Applicant claims priority based on Chinese Patent Application No.200810106323.3 filed 12 May 2008. The subject matter of theseapplications is incorporated herein.

TECHNICAL FIELD

This invention concerns the field of dangerous article detection, andmore particularly relates to method and system for detecting specialnuclear materials.

BACKGROUND ART

The so-called special nuclear materials refer to uranium and plutoniumthat can be used for the manufacture of nuclear weapons. It is generallyrequired that the abundance of the uranium 235 in uranium and theplutonium 239 in plutonium is at least 93%. Detection of special nuclearmaterials is detection of existence of uranium 235 and plutonium 239concealed in an enclosed environment (for example a vehicle orcontainer). Therefore, the special nuclear materials to be detectedhereinafter refer to uranium 235 and plutonium 239.

It is known that several kilograms to tens of kilograms of the specialnuclear materials are sufficient for constructing a simple nuclear bomb,thereby posing a great threat to social security. In some applications,the detection limit of special nuclear materials is defined as 100 cm³,which is very small compared with the enclosed environment (for examplea container) where special nuclear materials are possibly located. Inaddition, if the terrorists shield and camouflage the special nuclearmaterials to some extent, the detection difficulty thereof will befurther increased. Therefore, how to detect these special nuclearmaterials that are not in “large quantity” from the goods imported viavarious ports raises great challenge to the detection technology.

Current detection technology for special nuclear material is usuallyclassified as passive detection technology and active detectiontechnology.

The passive detection technology makes use of the spontaneousdisintegration phenomenon of the special nuclear materials. When theatomic nucleus of the special nuclear materials undergoes a spontaneousdisintegration, it will release prompt neutrons and γ-ray signals. It ispossible to find out the special nuclear materials bycollecting/detecting these ray signals by using a detector.

However, the intensity of the signals emitted when the special nuclearmaterials disintegrates spontaneously is weaker. Therefore, the passivedetection result is subject to the magnitude and shielding condition ofthe special nuclear materials and is very easy to be interfered by theshielding. If what measured by the detector is signal counting notenergy distribution, it is impossible to differentiate the signalemitted from the special nuclear material from that comes from theradiation background of the nature (for example γ-ray of potassium 40and neutrons produced by the cosmic rays). Therefore, the accuracy rateof detection is very low. Besides, the passive detection technologyneeds longer time to collect spontaneous disintegration signals and isnot suitable for the occasion that needs higher detection speed, forexample airport or harbour.

Nuclear resonance fluorescence technology is an active detectiontechnology, which makes use of electromagnetic radiation of specificenergy to irradiate the special nuclear materials. When the specificenergy and the atomic nucleus of the special nuclear materials have thesame energy level, strong absorption will occur. It is possible to carryout a complete definitive detection of the existence of the specialnuclear materials by the detecting absorption condition ofelectromagnetic radiation of the specific energy or detecting the energyof γ photons emitted from the object after it absorbs electromagneticradiation. This is a method having very good accuracy.

However, the nuclear resonance fluorescence technology needs to use adedicated accelerator to produce monoenergetic, high-energy X-rays. Inorder to produce monoenergetic X-rays of the magnitude of MeV toirradiate the object to be detected, it needs an electron acceleratormore than 100 MeV and high power laser source. At present, this kind ofray source is still in research phase and is not mature enough. Anotherray source is to directly make use of braking radiation source, and thenthe requirement on the electron accelerator is not high. It only needsto accelerate the electrons to the magnitude of MeV to 10 MeV. However,the measurement of the nuclear resonance fluorescence photons at thismoment is certainly accompanied with a lot background interference,which brings about great interference to the measurement and is adverse.

In a word, there appears no effective technology at present to detectspecial nuclear materials (in particular special nuclear materialsconcealed in an enclosed environment).

It is known that fissile materials, such as the special nuclearmaterials (namely uranium 235 and plutonium 239) and other nuclearmaterials (such as uranium 238 and plutonium 240) will undergo aphotofission under irradiation of X-rays. Further, the special nuclearmaterials will undergo a thermal neutron induced fission under theirradiation of thermal neutrons.

Very obviously, it is impossible to determine the special nuclearmaterials only by using photofission because it will be interfered byother fissile materials such as uranium 238 and plutonium 240. Thephotofission process cannot differentiate uranium 235 from uranium 238,nor can differentiate plutonium 239 from plutonium 240.

It is conceived to detect special nuclear materials only by thermalneutron induced fission. However, the yield of the fission productionproduced by the thermal neutron induced fission is smaller, whichreduces the sensitivity of detection of the special nuclear materialsand is easy to result in false alarm. If thermal neutrons are used toirradiate the special nuclear materials for a long time to accumulatesufficient fission signals, it will result in excessive long detectiontime, which is adverse in the occasions (such as Customs and harbour)that have requirement on detection speed. Besides, the thermal neutronsare hard to be collimated to a narrow region. Therefore, even if specialnuclear materials are found in the object to be detected, it is hard todetermine the position thereof.

SUMMARY OF THE INVENTION

Therefore, the object of this invention is to provide a method andsystem for detecting special nuclear materials. Said method and systemdetect the special nuclear materials by making use of the photofissioncharacteristic and the thermal neutron induced fission characteristicthereof. Such fission characteristics do not exist in other atomicnucleuses and thus can form features for detection of the specialnuclear materials.

According to one aspect of this invention, it provides a method fordetecting special nuclear materials, comprising:

irradiating a detection region of an object to be detected by using afirst X-ray beam, the energy of the first X-ray beam being selected toenable the special nuclear materials that possibly exist in the objectto be detected to undergo an observable light fission;

measuring a first fission ray signal emitted from the object to bedetected due to the photofission, where if the measured first fissionray signal exceeds a first threshold value, it is determined that thereexist fissile materials in the detection region of the objected to bedetected;

irradiating the detection region of the object to be detected with lowenergy neutrons when existence of fissile materials is determined, theenergy of the low energy neutrons being selected to enable the specialnuclear materials that possibly exist in the fissile material in theobject to be detected to undergo thermal neutron induced fission; and

measuring a second fission ray signal emitted from the object to bedetected due to the thermal neutron induced fission, where if themeasured second fission ray signal exceeds a second threshold value, itis determined that the fissile materials in the object to be detectedcontain special nuclear materials.

According to the method of this invention, it is possible to determinewhether the object to be detected contains fissile materials or not byusing X-ray beam through the photofission process. Further, since theX-ray beam has good collimatablility, it is possible to preciselydetermine the position of the fissile materials in the object to bedetected while determining the existence of the fissile materials,according to the method of this invention.

Since the process of detecting fissile materials by using photofissioncan be carried out more rapidly, and it does need the carry out thefollowing thermal neutron induced fission process if no fissilematerials are detected in the object to be detected, it is possible togreatly shorten the time used for the whole detection process. This verysuitable for the occasion that has requirement on detection speed.

It is possible to carry out the following thermal neutron inducedfission only after determining that fissile materials exist so as todetermine whether special nuclear materials exist or not. Since theexistence of the fissile materials is determined, it will notsignificantly increase the false alarm rate even if thermal neutrons areused to irradiate the object to be detected for shorter time. In thisway, compared with the method of only using thermal neutron inducedfission to detect special nuclear materials, the method of thisinvention can increase the detection speed and the detection accuracyrate.

In one preferred embodiment of this invention, the first fission raysignal may include a prompt fission ray signal and a delay fission raysignal emitted from the object to be detected due to photofission. Inthis way, it is possible to carry out double (repetitive) confirmationsof the existence of the fissile material by detecting the prompt fissionray signal during the photofission process and detecting the delayfission ray signal after the photofission process so as to increase theaccuracy of the detection.

In one preferred embodiment of this invention, the low energy neutronsmay be photoneutrons produced by bombarding a photoneutron conversiontarget with a second X-ray beam. In this way, it is possible to use thesame X-ray source (for example composed of an electron accelerator andan electronic target) to produce first and second X-ray beams so as toreduce the complexity and cost of the detection system.

In one preferred embodiment of this invention, the method furthercomprises detecting a suspicious region of the object to be detectedprior to irradiating with the first X-ray beam and taking the suspiciousregion as a detection region. It is known that besides the aforesaidfission characteristic, the special nuclear materials further have thecharacteristics of high density and high atomic number, for example theatomic numbers of uranium and plutonium are respectively 92 and 94, andthe densities thereof are respectively 18.95 g/cm³ and 19.84 g/cm³.These obviously exceed the atomic numbers and densities of otherconventional articles. Therefore, it is possible to take the regionhaving high density and/or high atomic number in the object to bedetected as the suspicious region. In this way, it is possible togreatly reduce the fission detection range.

In detecting the suspicious region, it is possible to use an X-raytransmission detection and/or neutron transmission detection method. TheX-ray beam and neutron beam used in the X-ray transmission detection andneutron transmission detection method can be the same as the X-raysource used in the photofission and thermal neutron induced fissionprocesses carried out above. This further simplifies the complexity ofthe whole detection system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exemplary embodiment of a system for detecting specialnuclear materials of this invention.

MODES OF CARRYING OUT THE INVENTION

FIG. 1 shows a detection system according to a preferred embodiment ofthe present invention. As shown in FIG. 1, said system comprises anelectron accelerator (not shown). Said electron accelerator can producean electron beam 1. The electron accelerator can preferably emit anelectron beam 1 of many kinds of energies (for example two kinds). Anelectronic target 2 is provided in the path of the electronic beam 1. Inone embodiment, the electronic target 2 is preferably composed oftantalum metal. An X-ray beam 3 is produced when the electronic beam 1bombards the electronic target 2.

The detection system in FIG. 1 further comprises a photoneutronconversion target 4, which is movable between a working position and anon-working position. The photoneutron conversion target 4 can be madeof heavy water, beryllium or depleted uranium. The photoneutronconversion target 4 in FIG. 1 is in the working position. At thismoment, the photoneutron conversion target 4 is in the travel path ofthe X-ray beam 3. In this way, the photoneutron conversion target 4 willproduce photoneutrons 5 when the X-ray beam 3 bombards the photoneutronconversion target 4. The photoneutron conversion target 4 in FIG. 1itself has a beam splitter, such that a part of the X-ray beam 3directly passes through the photoneutron conversion target 4 without anyaction therewith. Specifically, the beam splitter comprises a path inthe photoneutron conversion target 4 for the passage of a part of theX-ray beam 3. In this way, it is possible to produce X-rays andphotoneutrons simultaneously by using the photoneutron conversion target4. This part of X-ray beam 3 that directly passes through thephotoneutron conversion target 4 is collimated into an X-ray beam 9 byan X-ray collimator 6.

Such kind of photoneutron conversion target 4 with a beam splitter asshown in FIG. 1 is recorded in the Chinese patent application No.200810125197.6. Of course, it is possible to use other form ofcombination of a photoneutron conversion target and a beam splitter,such as the arrangement manner of the photoneutron conversion target andbeam splitter as recorded in the Chinese patent application No.200510086764.8. These two Chinese patent applications are cited hereinfor reference. Other independent photoneutron conversion target and beamsplitter are also considerable.

The photoneutron conversion target 4 in FIG. 1 can also be in thenon-working position (not shown). At this moment, the photoneutronconversion target 4 is out of the travel path of the X-ray beam 3. Inthis way, the X-ray beam 3 can directly irradiate the object 7 to bedetected without production of any photoneutrons. It is possible to usea simple pivoting structure to realize shift of the photoneutronconversion target 4 between the working position and the non-workingposition. When the photoneutron conversion target is in the non-workingposition, the X-ray beam 3 from the electronic target 2 is collimatedinto an X-ray beam 9 by the X-ray collimator 6.

The detection system in FIG. 1 further comprises a photoneutroncollimator 11 used for collimating the photoneutrons 5 produced into aphotoneutron beam 12.

The collimated X-ray beam 9 and photoneutron beam 12 can enter theobject 7 to be detected. The object 7 to be detected can be a vehicle orcontainer, wherein it contains special nuclear materials 8 to bedetected that possibly exist. Another side of the object to be detectedis provided with mutually spaced X/γ ray detector 10 and neutrondetector 13 used for detecting X/γ rays and neutrons emitted and/ortransmitted from the object 7 to be detected during carrying out thedetection process.

The method for detecting special nuclear materials of this invention ishereinafter further introduced with reference to FIG. 1.

1) Detecting a Suspicious Region in the Object to be Detected HavingHigh Density and/or High Atomic Number

The electron accelerator is adjusted such that it emits an electron beam1 of third energy (referred to as third electron beam in the claims).The photoneutron conversion target 4 is moved to its working position.The electron beam 1 bombards the electronic target 2 to produce an X-raybeam 3 (referred to as third X-ray beam in the claims). It needs to notethat the third X-ray beam 3 has a continuous energy spectrum.

A part of the X-ray beam 3 bombards the photoneutron conversion target4, thereby producing photoneutron conversion target 5. It needs to notethat the photoneutron beam 5 has a continuous energy spectrum. An Xneutron collimator 11 collimates the produced photoneutron beam 5 into aphotoneutron beam 12. Another part of the X-ray beam 3 passes throughthe path in the photoneutron conversion target 4 and forms into an X-raybeam 9 after collimated by the X-ray collimator 6.

The X-ray beam 9 is used to carry out X-ray transmission detection onthe object 7 to be detected. Specifically, the X-ray beam 9 enters theobject 7 to be detected and is disintegrated by substance including thespecial nuclear materials 8 that possibly exist. The X-rays transmittedfrom the object 7 to be detected enter the X/γ ray detector 10. The X/γdetector 10 detects the entering X-rays and forms correspondingelectrical signals. The amplitude of the electrical signals reflects thedisintegration information of the X-ray beam 9 in the object to bedetected. The X/γ ray detector 10 can be composed of a one-dimensionaldetector array disposed in height direction of the object to bedetected. Each measurement of the X/γ ray detector 10 can obtain aone-dimensional transmission data with respect to one cross section ofthe object to be detected. As the object 7 to be detected moves in thedirection as shown by an arrow 14, the X-ray beam 9 scan the object 7 tobe detected. In this way, it is possible to obtain a two-dimensionaltransmission data of disintegration of the object to be detected withrespect to the X-rays. Alternatively, it is possible to form an X-raytransmission image of the object 7 to be detected by using thetwo-dimensional transmission data.

While carrying out X-ray transmission detection on the object 7 to bedetected by using the X-ray beam 9, the photoneutron beam 12 is used tocarry out the following neutron transmission detection on the object 7to be detected. Specifically, the photoneutron beam 12 is projected intothe object 7 to be detected and is disintegrated by the substanceincluding the special nuclear materials 8 that possibly exist. Thephotoneutron beam 12 transmitted from the object 7 to be detected entersa neutron detector 13. The neutron detector 13 detects the enteringneutrons and form corresponding electrical signals. The amplitude of theelectrical signals reflects the disintegration signal of thephotoneutron beam 12 in the object 7 to be detected. The neutrondetector 13 can be composed of a one-dimensional detector array arrangedin height direction of the object to be detected. Each measurement ofthe neutron detector 13 can obtain a one-dimensional transmission datawith respect to one cross section of the object to be detected. As theobject 7 to be detected moves along the direction as shown by the arrow14, the photoneutron beam 12 scans the object 7 to be detected. In thisway, it is possible to obtain two-dimensional transmission data of thedisintegration of the object to be detected with respect to theneutrons. Alternatively, it is possible form neutron transmission imageof the object 7 to be detected by using the two-dimensional transmissiondata.

According to the X-ray transmission data and neutron transmission dataobtained above, it is possible to analyze the kind information of thematerials in the object to be detected by conventional means, therebydetermining the region having high atomic number therein. For example,in the Chinese patent application No. 200510086764.8, X-ray transmissiondata and neutron transmission data are used to form an n-x curve onlyassociated with equivalent atomic number Z, and the curve is used toidentify different materials within the object to be detected.

The present application provides another manner to identify the materialinformation in the object to be detected. Specifically, it is possibleto form a two-dimensional V value image of the object to be detected byusing the X-ray transmission data and neutron transmission data obtainedabove. The V value at each pixel point in the V value image (namely eachdetected point in the object to be detected) is defined as:V=In(I _(n) /I _(n0))/In(I _(x) /I _(x0))

wherein, I_(n0) represents the density of the incident neutron beam;I_(n) represents the density of the transmission neutron beam; I_(x0)represents the density of the incident X-ray beam; and I_(x) representsthe density of the transmission X-ray beam. The V value at each pixel isassociated with the kind of the material at the pixel. In this way,after obtaining the V value image of the object to be detected, it ispossible to determine the high atomic number region therein. The V valueimage can realize more sensitive detection of the specific materials inthe object to be detected.

Materials having high atomic number usually also have high density.

In other embodiments, it is also possible to determine the suspiciousregion in the object to be detected in other manners. For example, it ispossible to carry out X-ray transmission detection on the object to bedetected by using two X-ray beams in two perpendicular directions so asto determine the high density region therein.

Preferably, in determining the suspicious region, the X-ray beam and/orphotoneutron beam produced by bombarding the photoneutron target withX-ray beams are used. It is advantageous to use the same X-ray source(for example composed of an electron accelerator and an electronictarget) as that used in the process as will be described hereinafter soas to reduce the complexity and cost of the system.

If no high density and/or high atomic number region is found in theobject to be detected in this process, the whole detection process canbe ended and it is believed that no special nuclear materials exist inthe object to be detected.

2) Detecting Fissile Materials in the Suspicious Region

After a suspicious region is found, the photoneutron conversion target 4is moved to its non-working position. The object 7 to be detected ismoved by a driving system, such that the suspicious region is placed onthe detection position.

The electron accelerator is adjusted, such that it emits an electronbeam 1 (referred to as first electron beam in the claims) of firstenergy. The electron beam 1 bombards the electron target 2 to produce anX-ray beam 3 (referred to as first X-ray beam in the claims). The energyof the X-ray beam 3 should enable the special nuclear materials thatpossibly exist in the object 7 to be detected to undergo a photofission.Therefore, the first energy of the electron beam 1 is better to be notlower than 6.5 MeV, for example between 6.5 MeV and 15 MeV. In oneembodiment, the first energy can be selected as 9 MeV.

It needs to note that the third energy of the electron beam 1 used inthe aforesaid detection of the suspicious region can be the same as thefirst energy so as to reduce the requirement on the electronaccelerator. Of course, the third energy can also be different from thefirst energy and can be produced by a different electron accelerator.

The first X-ray beam 3 is collimated into an X-ray beam 9 by thecollimator 6. The X-ray beam 9 is used to irradiate the suspiciousregion. If fissile materials exist in the suspicious region of theobject 7 to be detected, they will undergo photofission underirradiation of the X first X-ray beam 3. The outcome of the photofissionincludes prompt γ-rays and neutron rays and delayed γ-rays and neutronrays (so-called β delay rays).

Since the electron beam 1 is usually electron beam impulse, the X-raybeam 4 is usually a series of X-ray pulses. In this way, it is possibleto use X/γ ray detector 10 and neutron detector 13 respectively tomeasure the prompt r-ray and neutron ray signals emitted from the object7 to be detected in the impulse interval between adjacent X-rayimpulses. If the measured densities of γ-rays and neutron rays exceedtheir environmental background levels, it proves that the suspiciousregion of the object 7 to be detected contains fissile materials.

Preferably, X/γ ray detector 10 and neutron detector 13 are further usedto measure β delay rays after stopping the irradiation of the X-ray beam3. If the measured β delay rays exceed the environmental backgroundlevel, it further confirms that the object to be detected surelycontains fissile materials.

In some other embodiments, it also possible to only measure the promptrays or the delay rays. In other embodiments, it is also possible toonly measure the γ-rays or the neutron rays. In yet other embodiments,as long as one kind of measurement values of the γ-rays and neutron raysexceeds its environmental background level, it proves that thesuspicious region of the object 7 to be detected contains fissilematerials.

If no fissile materials are found in the suspicious region in this step,the whole detection process can be ended and it is believed that nospecial nuclear materials exist in the object to be detected.

3) Detecting the Special Nuclear Materials

After existence of fissile materials is found, the photoneutronconversion target 4 is moved back to its working position. The electronaccelerator is adjusted, such that it emits an electron beam 1 (referredto as second electron beam in the claims) of second energy. It isappreciated that additional accelerators can also be used to produce theelectron beam 1 of second energy in other embodiments.

The second electron beam 1 bombards the electronic target 2 to producean X-ray beam 3 (referred to as second X-ray beam in the claims). Thesecond X-ray beam 3 bombards the photoneutron conversion target 4 toproduce photoneutrons. The photoneutrons are low energy neutrons and theenergy thereof is selected to enable the special nuclear materials thatpossibly exist in the fissile materials in the object to be detected toundergo a thermal neutron induced fission. Therefore, the second energyof the electron beam 1 is preferably between 2 MeV and 6 MeV, morepreferably between 3 MeV and 5 MeV. In one embodiment, the second energycan be selected as 4 MeV. The second energy is usually smaller than thefirst energy stated above. As far as the electron beam 1 having theselected second energy is concerned, the energy of the second X-ray beam3 produced thereby will not cause the fissile materials in the object 7to be detected to undergo a photofission, and therefore will nointerference with the following detection process about the thermalneutron induced fission.

The produced low energy neutrons produced are used to irradiate thesuspicious region of the object to be detected for a time. The lowenergy neutrons enter the suspicious region after moderated. If theobject 7 to be detected contains special nuclear materials, the lowenergy neutrons that enter the suspicious region cause it to undergo athermal neutron induced fission, thereby emitting γ-rays and neutronrays.

After stopping irradiation of the second X-ray beam 3 or low energyneutrons, the X/γ ray detector 10 and the neutron detector 13 are usedto measure the γ-rays and neutron rays emitted due to the thermalneutron induced fission. If the measured neutron rays and γ-rays exceedthe environmental background levels, it is possible to judge that thesuspicious region contains special nuclear materials, namely uranium 235and plutonium 239.

Although the foregoing embodiments include the step of detecting thesuspicious region, those skilled in the art can understand that the stepcan also be omitted and that the whole object to be detected is taken asa detection region for detection of the fissile materials therein insome other embodiments. In this occasion, it is possible to use thefirst X-ray beam 3 to scan the whole object 7 to be detected, therebydetermining the position of the fissile materials that possibly exist.

Although this invention has been described in connection with particularembodiments thereof, it should be noted that these embodiments are onlyillustrative, rather than limiting. Those skilled in the art can makealterations or variations to these embodiments within the range ofprotection defined by the appended claims.

The invention claimed is:
 1. A method for detecting special nuclearmaterials, comprising the steps of generating a first electron beam byan electron accelerator, the first electron beam having a first energy;producing a first X-ray beam using the first electron beam; irradiatinga detection region of an object to be detected with the first X-raybeam, the energy of the first X-ray beam being selected to enable thespecial nuclear materials that possibly exist in the object to bedetected to undergo an observable photofission; measuring a firstfission ray signal emitted from the object to be detected due to thephotofission, wherein if the measured first fission ray signal exceeds afirst threshold value, it is determined that the detection region of theobject to be detected contains fissile materials; generating a secondelectron beam by the electron accelerator if it is determined that thedetection region of the object to be detected contains fissilematerials, the second electron beam having a second energy differentthan the first energy; producing a second X-ray beam by bombarding anelectronic target with the second electron beam, wherein the energy ofthe second X-ray beam is lower than the energy of the first X-ray beamand the second X-ray beam is used to generate low energy neutrons;irradiating the detection region of the object to be detected with thelow energy neutrons, the energy of the low energy neutrons beingselected to enable the special nuclear materials that possibly exist inthe fissile materials in the object to be detected to undergo a thermalneutron induced fission; measuring a second fission ray signal emittedfrom the object to be detected due to the thermal neutron inducedfission, wherein if the measured second fission ray signal exceeds asecond threshold value, it is determined that the fissile materials inthe object to be detected contain special nuclear materials.
 2. Themethod according to claim 1, wherein the first fission ray signalincludes: a prompt fission ray signal and/or delay fission ray signalemitted from the object to be detected.
 3. The method according to claim2, wherein the first X-ray beam includes a series of X-ray impulses, andmeasuring of the prompt fission ray signal is carried out in the impulseinterval between adjacent impulses of the first X-ray beam.
 4. Themethod according to claim 2, wherein measuring of the delay fission raysignal is carried out after stopping the first X-ray beam.
 5. The methodaccording to claim 2, wherein measuring of the first fission ray signalincludes not only measuring of the prompt fission ray signal but alsomeasuring of the delay fission ray signal so as to carry out doubleconfirmations of the existence of the fissile materials.
 6. The methodaccording to claim 1, wherein the energy of the second X-ray beam isselected to be lower than the energy that enables the fissile materialsto undergo an observable photofission.
 7. The method according to claim1, wherein measuring of the second fission ray signal is carried outafter stopping the second X-ray beam.
 8. The method according to claim1, wherein the first and second X-ray beams are respectively produced bybombarding the electronic target with first and second electron beamshaving different energies, and the energy of the first electron beam ishigher than that of the second electron beam.
 9. The method according toclaim 1, wherein the first and second fission ray signals include γ-rayand/or neutron ray signals respectively.
 10. The method according toclaim 9, wherein the first and/or second threshold values are selectedas the environmental backgrounds of the γ-rays and/or neutron rays. 11.The method according to claim 1, further comprising the steps of:detecting the suspicious region of the object to be detected; and takingthe suspicious region as a detection region prior to irradiating withthe first X-ray beam, the suspicious region including a high densityand/or high atomic number region in the object to be detected.
 12. Themethod according to claim 11, wherein the detecting of the high atomicnumber region includes: carrying out X-ray transmission detection andneutron transmission detection on the object to be detected anddetermining the high atomic number region in the object to be detectedwith the obtained X-ray transmission data and neutron transmission data.13. The method according to claim 12, wherein the X-ray transmissiondetection uses a continuous energy spectrum X-ray beam; the neutrontransmission detection uses a continuous energy spectrum neutron beam;and the X-ray transmission data and the neutron transmission data areused to form a V value image of the object to be detected, wherein the Vvalue at each pixel in the V value image of the object to be detected isdefined as:V=In(In/In0)/In(Ix/Ix0) wherein, In0 represents the intensity of theincident neutron beam; In represents the intensity of the transmissionneutron beam; Ix0 represents the intensity of the incident X-ray beam;and Ix represents the intensity of the transmission X-ray beam, whereinthe V value at each pixel is associated with the kind of the materialsat the pixel.
 14. The method according to claim 13, wherein thecontinuous energy spectrum X-ray beam is formed by a part of a thirdX-ray beam, and the continuous energy spectrum neutron beam is formed bybombarding a photoneutron conversion target with another part of thethird X-ray beam.
 15. The method according to claim 14, wherein thethird X-ray beam and the first X-ray beam are respectively produced bybombarding an electronic target with the third and the first electronbeams.
 16. The method according to claim 15, wherein the third and firstelectron beams have the same energy.
 17. The method according to claim15, wherein the third and second electron beams are produced by the sameelectron accelerator.
 18. The method according to claim 8, wherein theamount of energy of the first electron beam is selected to be not lowerthan 6.5 MeV and the amount of energy of the second electron beam ispreferably between 2 MeV and 6 MeV.
 19. The method according to claim18, wherein the amount of energy of the first electron beam is selectedto be between 6.5 MeV and 15 MeV and the amount of energy of the secondelectron beam is more preferably between 3 MeV and 5 MeV.